I. Field of the Invention
This invention relates generally to an X-ray focusing system. In particular, the present invention relates to a Multilayer Laue Lens (MLL) configured to focus hard X-rays in a transmission geometry and to a fabrication method of making an MLL.
II. Background of the Related Art
Modern synchrotron-radiation facilities provide unprecedented levels of intensity and collimation in X-ray beams and offer tremendous research opportunities. The development of improved X-ray focusing optics is essential for further advances in various areas such as X-ray microimaging and microanalysis applications.
Focusing optics for X-rays differ from those for visible light, as the refractive index of solids is slightly smaller than unity for X-rays and significantly greater than unity for visible light. Reflective X-ray mirrors, such as elliptical Kirkpatrick-Baez (KB) mirrors and tapered hollow capillaries, can be used only at very small grazing angles below the critical angle of the reflecting material. Moreover, hard X-rays, those with wavelengths shorter than 0.1 nm, are notoriously hard to focus. Traditional lenses fail to bend hard X-rays because their index of refraction is very close to unity in this part of the spectrum. Even smooth surfaces reflect hard X-rays at only an extremely glancing angle.
Traditional zone plates for X-ray focusing optics are fabricated using lithographic techniques with metal electroplating on silicon nitride membranes. For efficient focusing of hard X-rays, a very large aspect ratio is required, which presents a formidable challenge for the manufacturing process. To achieve a high aspect ratio of zone depth to width, a mask with the zone-plate pattern is first made using e-beam lithography. X-ray lithography is then used, with a thick photoresist and subsequent metal electroplating on silicon nitride membranes, for zone-plate fabrication. Tremendous progress has been made in this field, and very recently, a spatial resolution of 60 nm was achieved for 8 keV hard X-rays, using zone plates with a 50 nm outermost zone width and 1 μm zone depth with gold as the zone material. However, as the desired zone width becomes smaller and zone depth larger, the manufacturing process becomes increasingly difficult.
Researchers at UChicago Argonne, LLC (Chicago, Ill., US) designed a non-traditional type of lens that uses diffraction to focus the high-energy beams into a tiny region. The device is called a multilayer Laue lens (MLL), because it diffracts X-rays in a transmission geometry, rather than a reflection geometry. An exemplary MLL was made out of 728 layers of silicon (Si) and tungsten silicide (WSi2) sputtered onto a silicon wafer substrate. See, e.g., U.S. Pat. No. 7,440,546 B2 to Liu et al., the disclosure of which is incorporated by reference in this specification.
FIG. 1 of the present specification illustrates cross sections of the multilayer sections of different types of Multilayer Laue Lens known in the art such as (a) a flat-type MLL 102, (b) a wedge-type MLL 104, and (c) a tilt-type MLL 106 (c). Each of the MLLs of FIG. 1 is a multilayer device configured to focus hard X-rays by X-ray diffraction, and obeys the zone-plate law, while having a different shape and fabrication method, as compared to the traditional zone-plates. Each of the MLLs is formed with substantially identical first and second multilayer sections formed with a plurality of alternating layers of selected materials. It is also noted that each of the multilayer sections has a monotonically increasing thickness from a minimum thickness adjacent the outer most zone and a maximum thickness of near the central portion nearest to an optical axis (OA) of the device.
It has been known that the optical performance of the MLLs strongly depend on the angle of the layers with respect to the incident beams. In particular, the wedge-type MLL is known to show a better focusing efficiency than other type of MLLs, such as the flat-type and tilt-type MLLs, because each of the layers in the multilayer sections of the wedge-type MLL satisfies the Bragg condition, θB≈λ/2Λ and Λ (rn)≡rn−rn-2≈λf/rn, where rn is the distance from the nth interface to the optical axis and f is the focal length, so that the focusing efficiency of the lens is improved.
However, in order to obtain the in-phase interference of radiation at the focus, each of zone-pair from both halves of an MLL must be separated from the center of the device by a predetermined distance. Specifically, an individual layer placement error should be less than approximately one-third of the thickness of that particular layer, and this rule applies to all of the layers in the multilayer sections of the device stack. Because the thickness of layers in the multilayer sections monotonically decreases in the MLL, as the distance of the layers in question to the center of the MLL increases, the allowable error in an absolute placement distance also decreases as the distance of the layers in question to the center of the MLL increases. For the inner zones where the thickness of the layers is relatively thicker, satisfying the placement requirement may be reasonable. However, meeting the placement requirement may be extremely difficult for the outermost zones not only because the thickness of the layers is relatively thinner, but also the placement errors of all previous zones are accumulated.
For example, assuming that the illumination area spans over 100 μm in the horizontal direction of an MLL, and the minimum thickness of the outermost zone of the MLL is 1 nm, the separation uniformity of the outermost zone-pair must be within 3 angstrom (Å) over the 100 μm illumination range. However, due to accumulated errors and system drifts during the growth of the multilayer thin films, this is an increasingly-difficult criterion to meet, especially when the thickness of the outermost zones approaches the sub-nanometer scale.
Accordingly, it is an objective to provide a structure of a MLL device, and an apparatus and method of fabricating the MLL device that can effectively address the disadvantages and problems associated with conventional MLL device structures.
Another goal is to eliminate the strict requirement of zone compensation placement error inherent in the fabrication process of the prior MLL device structures.